1. Field of the Invention
The present invention relates to a high-density magneto-optical recording medium.
2. Description of the Related Art
A magneto-optical disk is known as a high-density recording medium, and an increase in information quantity gives rise to a desire for higher densities of the medium. While the higher densities may be realized by reducing the space of recorded marks, the recording and reproducing of the marks are limited by the size of a light beam (beam spot) on the medium. When the presence of only one recorded mark in the beam spot is set, an output waveform corresponding to "1" or "0" may be observed as a reproduction signal according to whether the recorded mark is present or absent in the beam spot.
However, when the presence of plural recorded marks in the beam spot is set by reducing the space of the recorded marks, no change in reproduction output occurs regardless of movement of the beam spot on the medium. Accordingly, the output waveform becomes linear and the presence or absence of the recorded mark in the beam spot cannot be identified. To reproduce such small recorded marks having a period smaller than the size of the beam spot, it is sufficient to reduce the beam spot to a small size. However, since the size of the beam spot is limited by the wavelength .lambda. of a light source and the numerical aperture NA of an objective lens, the beam spot cannot be sufficiently reduced to a small size.
There has recently been proposed a reproducing method using magnetically induced super resolution such that a recorded mark smaller in size than the beam spot can be reproduced by the use of an existing optical system. According to this method, the resolution of reproduction is improved by masking other marks during reproduction of one mark in the beam spot. Accordingly, a super resolution disk medium is required to have at least a mask layer or a reproducing layer for masking other marks so that only one mark may be reproduced during signal reproduction, in addition to a recording layer for recording marks.
Japanese Patent Laid-open No. Hei 3-93058 discloses a method of increasing a reproducing resolution with a magneto-optical recording medium having a nonmagnetic intermediate layer interposed between a reproducing layer and a recording layer both formed from a perpendicular magnetization film. However, the reproducing method described in this publication has a problem such that when a reproducing power is increased, a recorded mark transfer region is widened to cause a decrease in reproduction output. As another conventional method, Japanese Patent Laid-open No. Hei 4-271039 discloses an information reproducing method using a medium having a magnetic intermediate layer interposed between a reproducing layer and a recording layer to form two mask regions in a beam spot, thereby maintaining a high resolution to eliminate a decrease in reproduction output in spite of an increase in reproducing power. However, the information reproducing method described in this publication has a problem such that it is necessary to initialize the reproducing layer by using an initializing magnetic field of several kilo-oersteds (kOe) after recording information on the medium, and that it is also necessary to apply a reproducing magnetic field of 200 oersteds (Oe) or more in order to form the two mask regions in the beam spot.
As mentioned above, in the conventional magnetic super resolution magneto-optical disk medium having the nonmagnetic intermediate layer interposed between the reproducing layer and the recording layer to transfer information recorded in the recording layer to the reproducing layer by magnetostatic bond, an increase in reproducing power causes a decrease in reproduction output. Further, in the other conventional magnetic super resolution magneto-optical disk medium having the magnetic intermediate layer interposed between the reproducing layer and the recording layer to form masks for preventing a decrease in reproduction output in the beam spot, a large reproducing magnetic field of 200 Oe or more and a large initializing magnetic field of several kOe are required. In an ordinary magneto-optical disk unit currently used, no magnetic field is applied in reproducing information. Accordingly, information recorded on the magneto-optical disk unit cannot be reproduced by the ordinary magneto-optical disk unit.